format binary as "XXX"
use16
org 0

virtual at $1000
  xx2: db ?
  ; xxx: db ?
  xx3: db ?
  xx4: db ?
end virtual

virtual at bx
  bx2: db ?
  ; bxx: db ?
  bx3: db ?
  bx4: db ?
end virtual

mov ax, 0

mov [xx4], al
mov [xx2], ax ; ZERO'ize both "xx2" and "xx3"
mov [xx4], ax ; BUG but fine for the compiler

if ((xx2+1)<>xx3)
 err "(xx2+1)<>xx3"
end if

mov [bx4], al
mov [bx2], ax ; ZERO'ize both "bx2" and "bx3"
mov [bx4], ax ; BUG but fine for the compiler

; if ((bx2+1)<>bx3)
;  err "(bx2+1)<>bx3"
; end if
    

 if ((bx2+1)-bx3<>0)
  err "(bx2+1)<>bx3"
 end if    

assert (bx2+1-bx3=0)    

This seems to be a nice idea.

macro assert [expr]
{
       common
              if ~(expr)
                  display 'Error: assertion failed',13,10
                       rb -1
               end if
}    

macro irpm [value]
{
        ;do something
}
irpm x,y,z    

irp value,x,y,z
{
 ;do something
}    

Why make an additional compiler directive, if the same can be easily achieved with macros?

Code:

macro assert [expr]
{
   common
              if ~(expr)
                  display 'Error: assertion failed',13,10
                       rb -1
               end if
}    

If it's needed to stop the assembly only when the condition is permanent, and not just occuring in the intermediate assembly passes, the trick with rb -1 can be used instead. The rb directive does not cause an error when it is provided with negative value in the intermediate passes.

Code:

macro assert [expr]
{
      common
              if ~(expr)
                  display 'Error: assertion failed',13,10
                       rb -1
               end if
}    

assert (bx2+1-bx3=0)
    

Assert directive, condition is false, bx2 is "bp+4", bx3 is "bp+6"
    

You macro doesn't show the offending condition.

even better, assert could display the values involved in the false expression

macro static_assert cond, message
{
  local ..static_assert_pass
  if defined ..static_assert_pass & ~ (cond)
    if ~ message eq
      display "Assertion failed: ", message, 13, 10
    end if
    err
  end if
  ..static_assert_pass:
}    

The "assert" has been added in 1.69.36 release.